1. Field of the Invention
This invention relates to a photosensitive recording material and a photosensitive recording medium that are used to form a volume type phase hologram, and a process for producing a hologram using such a photosensitive recording medium. More particularly, it relates to a photosensitive recording material and a photosensitive recording medium that have a high sensitivity to visible light, in particular, to argon laser light and electron rays, also have superior weatherability and storage stability and have good hologram characteristic values for, e.g., resolution, diffraction efficiency and transparency, and a process for producing a hologram using such a photosensitive recording medium.
2. Description of the Prior Art
Holograms enable reproduction of three-dimensional stereoscopic images, and hence have been hitherto used in covers of books, magazines or the like, pop art display, gifts and so forth on account of their attractive designability and decorative effect. Holograms can also be said to be equivalent to records of information in submicroscopic units, and hence they are also used as marks for preventing forgery of marketable securities, credit cards and so forth.
In particular, in volume type phase holograms, spatial interference fringes with differences not in optical absorption but in refractive indexes are formed in photosensitive recording mediums, whereby phases can be modulated without absorption of light beams passing through images. Hence, in recent years, they are not only used for display but also expected to be applied in hologram optical elements (HOE) as typified by head-up display (HUD) on the windshield of cars.
Now, recording materials for forming the volume type phase holograms are required to be highly sensitive to laser light having visible oscillation wavelength and besides to show a high resolution. When actually used in forming holograms, they are also required to provide holograms having characteristics such as diffraction efficiency, wavelength reproducibility of reproducing light, band width (half width of a peak of reproducing light) and so forth suited for their purposes. In particular, recording mediums for HUD holograms should preferably have the properties that the diffraction efficiency is 90% or more at spatial frequency of 5,000 to 6,000 lines/mm, the half width of a peak of reproducing light (the band width) is 20 to 30 nm and the peak wavelength of reproducing wavelength is within 5 nm of photographing wavelength, and are also required to have a good storage stability over a long period of time.
The general principle concerning the production of holograms is described in some publications and technical books, for example, Junpei Tsujinai, "Holographic Display", Chapter 2, Sangyo Tosho Co. According to these, one beam of a coherent optical system with dual light fluxes, which is commonly a laser, is directed to a recording object, and a photosensitive recording medium as exemplified by a photographic dry plate is placed at a position where the total-reflected light can be received. In addition to the beam reflected from the object, another coherent beam is directed into the recording medium directly without striking the object. The beam reflected from the object is called the object beam, and the beam directed to the recording medium, the reference beam. Interference fringes composed of the reference beam and the object beam are recorded as image information (a hologram). Next, the recording medium having been processed is irradiated by light and viewed at a suitable position, where the light from an illumination source is diffracted by the hologram so as to reproduce the wave front of the reflected light having first reached the recording medium from the object at the time of recording. As a result, an object image similar to an actual image of the object is three-dimensionally seen. Holograms formed by making the object beam and reference beam incident on the recording medium from the same direction are known as transmission holograms. In contrast thereto, holograms formed by making them each other incident from the opposite side of the recording medium are commonly known as reflection holograms. The transmission holograms can be obtained by known methods as disclosed in, for example, U.S. Pat. No. 3,506,327 and No. 3,894,787. The reflection holograms can be obtained by known methods as disclosed in, for example, U.S. Patent No. 3,532,406.
As a value for comparing holographic characteristics of holograms formed as images, refractive index modulation is used. This is a value calculated from the measured diffraction efficiency and recording medium thickness, the former being the proportion of incident light diffracted by a diffraction grating which is prepared while directly irradiating a recording medium in the manner that dual light fluxes are at the same angles to the recording medium. The refractive index modulation is a quantitative measure of the changes in refractive index that occur at exposed areas and unexposed areas of a volume hologram, i.e., the portions where light rays interfere with one another to become strong or weak in intensity, and can be found by the Kogelnik's theoretical formula (Bell. Syst. Tech. J., 48, 2909, 1969). In general, the reflection holograms have more interference fringes formed per 1 mm than the transmission holograms and hence make it difficult to carry out recording, so that it is difficult to obtain a high refractive index modulation.
As recording materials for such volume type phase holograms, photosensitive materials of bleached silver salt and dichromated gelatin types have been hitherto commonly used. The dichromated gelatin type photosensitive materials are materials most widely used in the recording of volume type phase holograms, because of their high diffraction efficiency and low noise characteristics. Such photosensitive materials, however, have so short a storage lifetime that they must be prepared every time the holograms are produced. Also, since the development is carried out by the wet process, holograms may undergo deformation in the course of swell and shrink of the gelatin required when holograms are produced. Hence, such materials have the problem that holograms have a poor reproducibility. As for the silver salt photosensitive materials, they require complicated processing after recording, and they are photosensitive materials that cannot be satisfactory in view of stability and workability. These aforesaid photosensitive materials also all have the problem that they have inferior environmental properties as exemplified by humidity resistance and weatherability.
To overcome such problems, as materials having superior environmental properties and other properties to be possessed by hologram recording materials, such as a high resolution and a high diffraction efficiency, hologram recording materials making use of poly-N-vinylcarbazole are known in the art. For example, a hologram recording material comprising a cross-linking agent cyclic cis-.alpha.-dicarbonyl compound and a sensitizer (Japanese Patent Application Laid-open No. 60-45283), a hologram recording material comprising a 1,4,5,6,7,7-hexachloro-5-norbornene-2,3-dicarboxylic anhydride and a dye (Japanese Patent Application Laid-open No. 60-227280), a hologram recording material comprising 2,3-bornanedione and Thioflavine (Japanese Patent Application Laid-open No. 60-260080), a hologram recording material comprising Thioflavine T and iodoform (Japanese Patent Application Laid-open No. 62-123489) and so forth are proposed. Since, however, these hologram recording materials still also require the wet-process development, they require complicated processing steps and have the problem of a poor reproducibility. Since also they are photosensitive materials mainly composed of poly-N-vinylcarbazole, though being chemically stable and having a high resolution and superior environmental properties, the poly-N-vinylcarbazole tends to turn white upon crystallization, and has the problems that they have a poor reproducibility of transparency and solvents are limited. In addition, they are desired to be more improved in view of sensitivity characteristics.
As recording materials capable of being photocured at a high sensitivity, a photocuring resin composition used in combination of a 3-ketocoumarin dye with a diaryl iodonium salt which are constituents of a photopolymerization initiator (Japanese Patent Application Laid-open No. 60-88005) and also a hologram recording material comprised of such a photopolymerization initiator and a support polymer poly(methyl methacrylate) in combination (Japanese Patent Application Laid-open No. 4-31590) are proposed. These recording materials are chemically stable and have a high resolution and a high sensitivity, but are accompanied by formation of holes or pores on account of wet processing. Hence, they have the problems that the peak wavelength of reproducing wavelength becomes non-uniform, the half width of the peak wavelength expands and also, when developed, uneven development tends to occur because of a more or less resolution of supporting polymers in swelling solvent, and still also the presence of a large number of holes or pores in the hologram results in poor thermal resistance and thermopressure resistance.
As a measure for overcoming such problems, photopolymerization type photosensitive materials that enable production of a hologram through a sole processing step without any wet processing are disclosed in U.S. Pat. No. 3,993,485 and No. 3,658,526. The former discloses two types of photosensitive materials. A first example is a photosensitive resin composition comprised of combination of i) two polymerizable ethylenically unsaturated monomers having different reactivities and refractive indexes with ii) a photopolymerization initiator, as exemplified by a cyclohexyl methacrylate, N-vinylcarbazole and benzoin methyl ether system, which is held between two glass sheets, followed by exposure using a dual light flux optical system to record a hologram. A second example is a photosensitive resin composition comprised of four components, i.e., a polymerizable ethylenically unsaturated monomer and an ethylenically unsaturated monomer acting as a cross-linking agent when the former is polymerized, both having substantially the same degree of refractive index, a non-reactive compound having a different refractive index than the two monomers, and a polymerization initiator, as exemplified by a butyl methacrylate, ethylene glycol dimethacrylate, 1-phenylnaphthalene and benzoin methyl ether system, which can produce a hologram in the same manner as the first example. Whichever photosensitive resin compositions are used, the polymerization of monomers having higher reactivity proceeds at areas where the interference fringes formed by the dual light flux optical system have a strong light intensity and at the same time the density gradation occurs in monomers to cause the monomers with a high reactivity to be diffused in the areas with a strong light intensity and cause the monomers with a low reactivity or non-reactive compounds to be diffused in the areas with a weak light intensity. Thus, the interference fringes are recorded according to differences in refractive indexes to form a volume type phase hologram.
However, such hologram recording photosensitive resin compositions have had the following problems. In the composition shown in the first example, the monomers with a low reactivity undergo polymerization to a certain degree, and no high refractive index modulation can be obtained. In the second example, the non-reactive compound 1-phenylnaphthalene is present in the system as a compound with a low molecular weight even after the hologram has been finished, resulting in no storage stability. Also, in both the examples, since they are mixtures having a low molecular weight and have a low viscosity, they can be held between substrates with difficulty or can form thick films with difficulty, having many problems on workability and reproducibility.
As for the latter U.S. Pat. No. 3,658,526, it discloses a process for producing a stable hologram formed of a hologram recording material comprising a polymer matrix incorporated with a photopolymerizable ethylenic monomer and a photopolymerization initiator, according to which a permanent volume type phase hologram can be obtained by one-time exposure to actinic radiation. The hologram thus formed is fixed by subsequent overall irradiation with actinic radiation. The hologram recording material disclosed in that publication aims at many advantages in view of workability and reproducibility, but has a low diffraction efficiency. In this hologram recording material, the hologram finished has a refractive index modulation ranging from 0.001 to 0.003. As a result, the reproduced images of the hologram formed can only have a limited brightness. The brightness may possibly be improved to a certain extent by increasing the thickness of the hologram recorded layer. This measure to solve the problem, however, consequently forces manufacturers to use hologram recording materials in a large quantity, and also causes a difficulty when holograms are used as fixtures in laminated safety glass as in HUD on the windshield of cars. It should be also noted that the holograms obtained by this process usually cause a decrease in diffraction efficiency after storage for a long time.
Now, improvement techniques including the production of hologram recording materials disclosed in this U.S. Pat. No. 3,658,526 are disclosed in U.S. Pat. No. 4,942,112 and No. 5,098,803. These publications disclose a composition basically consisting of a thermoplastic resin, a polymerizable ethylenically unsaturated monomer and a photopolymerization initiator, where a compound having an aromatic ring is used in either the thermoplastic resin or the polymerizable ethylenically unsaturated monomer in order to improve refractive index modulation, so as to provide a difference in refractive index. Since, however, similar to what is disclosed in U.S. Pat. No. 3,658,526, a resin with a high molecular weight is used as a binder matrix, there is a limit on the diffusibility of monomers at the time of exposure, so that a large amount of exposure becomes necessary and also no high diffraction efficiency can be obtained. To eliminate this disadvantage, a non-reactive plasticizer is added. The use of such a plasticizer, however, causes a problem on the film strength of the hologram formed, and also such a non-reactive plasticizer is present in the system as a compound with a low molecular weight even after the hologram has been finished, resulting in no storage stability. In addition, since the carrier that holds the monomers and so forth is a thermoplastic resin, there is a disadvantage that the hologram has a poor thermal resistance.
As a proposal to eliminate such a disadvantage, Japanese Patent Application Laid-open No. 5-107999 discloses a recording material in which the plasticizer disclosed in the above patent is replaced with a cationic polymerizable monomer and a cationic polymerization initiator so that the problems caused by the non-reactive plasticizer remaining after the formation of holograms can be solved.
This recording material, however, requires a reasonable irradiation with light to fix the hologram after its formation, and also, at the time of fixing, the hologram formed may cause a strain because of diffusion of the cationic polymerizable monomer with a low molecular weight to make it impossible to obtain a high diffraction efficiency. Since also, similar to the prior art thereof, the carrier that holds the monomers and so forth is a thermoplastic resin, there is a disadvantage that the hologram has a poor thermal resistance. Moreover, in a system where no resin binder is used as the carrier for holding them, the recording material can be held between substrates with difficulty because of a low viscosity or can form thick films with difficulty, having many problems on workability and reproducibility.
Under such technical backgrounds, Japanese Patent Application Laid-open No. 5-94014 discloses, as an improvement of the recording materials disclosed in the above U.S. Pat. No. 4,942,112 and No. 5,098,803 and Japanese Patent Application Laid-open No. 5-107999, a hologram photosensitive resin composition comprised of an epoxy resin, a radical polymerizable ethylenically unsaturated monomer and a radical photopolymerization initiator.
So far as seen in Examples disclosed in the above Japanese Patent Application Laid-open No. 5-94014, the hologram photosensitive resin composition makes use of two kinds of epoxy resins. When, however, ultraviolet-curing epoxy resin is used, troublesome operations are required such that the radical polymerization and the cationic polymerization are carried out under light with different wavelength regions, and also, in order to control the diffusibility of monomers, a microadjustment control is required such that the viscosity is increased by pre-exposure. Thus, this composition still has the problem of difficulties in workability and reproducibility. When thermosetting epoxy resin and a curing agent are used, it takes a reasonable ultraviolet-curing and heating time to cure the epoxy resin for the fixing, resulting in a very poor workability. In addition, the improvement technique disclosed in this publication has a great problem that no high diffraction efficiency can be obtained.
As discussed above, the photopolymerization type photosensitive materials that enable production of a hologram by the sole processing step without any wet processing have the problem on polymerizability and diffusibility of monomers for obtaining a high refractive index modulation and the problem on storage stability caused by the addition of the monomer-holding carrier and the non-reactive additive. In addition, they cannot obtain photosensitive recording materials and photosensitive recording mediums having a good workability for producing holograms and good holographic performances such as hologram diffraction efficiency, transparency and reproducibility. Thus, it is still sought to provide a photopolymerizable composition improved for the hologram recording. In particular, it can be said to be natural to do so with regard to HOEs produced using the same.